The invention concerns a switchable infrared filter, which without moving parts can be switched from an infrared-transparent state into an infrared-opaque state. Such infrared filters can be advantageously used to modulate a light beam. A further application lies in the field of switchable protective filters, for example, for the protection of sensitive optical sensors from laser radiation.
From U.S. Pat. No. 3,484,722 it is of known art to use a single crystal of vanadium dioxide as a switchable infrared filter. According to the teaching of this document vanadium dioxide is a thermochromic material, which at approx 68° C. demonstrates a semiconductor-metal transition. This phase transition effects a change from an infrared-transparent state to an infrared-opaque state. Particularly disadvantageous here is that the relatively thick crystal can only be heated and cooled slowly such that the switching processes in each case occupy a long period of time. Moreover the infrared filter can only be operated in the reflection mode since the transmission of a light beam is prevented by the heating device fitted at the rear.
A further development of this infrared filter is demonstrated by U.S. Pat. No. 4,795,240. Here the thermochromic vanadium dioxide is applied as a coating onto a silicon wafer. An insulation layer of silicon oxide is located between the thermochromic material and the silicon. Heating of the thermochromic material can take place in a simple manner by means of a direct flow of current through the silicon wafer.
Also disadvantageous in this solution, however, is that on account of the large thermal mass and the low thermal conductivity the filter can be switched only very slowly between the infrared-transparent state and the infrared-opaque state. Furthermore in thermal equilibrium the heated filter has a very non-homogeneous temperature distribution. This leads to spatially non-homogeneous transmission characteristics.
Also with this filter of previous known art operation in transmission mode is only possible to a limited extent. The reason for this lies in particular in that doped silicon material with sufficient electrical conductivity forms impurities, which as a result of incident infrared radiation can also be ionised with photon energies smaller than the band gap energy. As a result even in the infrared-transparent state of the vanadium dioxide there is undesired absorption of the infrared radiation passing through the crystal.
The object of the present invention accordingly consists in providing an infrared filter with faster response characteristics, which has a homogeneous temperature over a sufficiently large surface, and thus has homogeneous transmission characteristics. Furthermore the object consists in specifying a switchable infrared filter, which has an enhanced transmission mode.